1. Field of the Invention
This invention relates to a reception apparatus, a reception method and a reception system, and particularly to a reception apparatus, a reception method and a reception system which can implement improvement of the stability and the noise-resisting performance upon starting of reception even where a signal which includes a signal other than a T2 frame is to be received.
2. Description of the Related Art
In terrestrial digital broadcasting and so forth, orthogonal frequency division multiplexing (OFDM) is adopted as a modulation method for data.
In the OFDM, a large number of orthogonal subcarriers are provided in a transmission band, and digital modulation such as phase shift keying (PSK) or quadrature amplitude modulation (QAM) wherein data are allocated to the amplitude or the phase of the subcarriers is carried out.
In the OFDM, since data allocation to a plurality of subcarriers is carried out, modulation can be carried out by IFFT (Inverse Fast Fourier Transform) operation by which inverse Fourier transform is carried out. Further, demodulation of an OFDM signal obtained as a result of the modulation can be carried out by FFT (Fast Fourier Transform) operation by which Fourier transform is carried out.
Accordingly, a transmission apparatus which transmits an OFDM signal can be configured using a circuit which carries out IFFT operation, and a reception apparatus which receives an OFDM signal can be configured using a circuit which carries out FFT operation.
Further, in the OFDM, data are transmitted in a unit called OFDM symbol.
An OFDM symbol is generally configured from an effective symbol which is a signal period within which IFFT is carried out upon modulation, and a guard interval in which a waveform of part of a rear half of the effective symbol is copied as it is at the top of the effective symbol. By providing the guard interval at the top of the OFDM symbol in this manner, the resisting property to multipath noise can be improved.
Further, in the OFDM, a pilot signal which is a known signal, that is, a signal known to the reception apparatus side, is discretely inserted in the time direction or the frequency direction, and a reception side utilizes the pilot signal for synchronization, estimation of a transmission line characteristic and so forth.
It is to be noted that, according to terrestrial digital broadcasting standards which adopt the OFDM, a unit called frame, that is, OFDM transmission frame, configured from a plurality of OFDM symbols, is defined, and transmission of data is carried out in a unit of a frame.
The reception apparatus for receiving such an OFDM signal as described above uses a carrier of the OFDM signal to carry out digital orthogonal demodulation of the OFDM signal.
However, generally the carrier of the OFDM signal used for digital orthogonal demodulation by the reception apparatus does not coincide with the carrier of an OFDM signal used in the transmission apparatus from which the OFDM signal is transmitted but includes some errors. In other words, the frequency of the carrier of the OFDM signal used for digital orthogonal demodulation is displaced or offset from the center frequency of an IF (Intermediate Frequency) signal of the OFDM signal received by the reception apparatus.
Therefore, the reception apparatus carries out a carrier displacement amount detection process of detecting a carrier displacement or offset amount which is an error of the carrier of the OFDM signal used for digital orthogonal demodulation and a correction process, that is, an offset correction process of correcting the OFDM signal in accordance with the carrier displacement amount so as to eliminate the carrier displacement amount.
As one of standards for terrestrial digital broadcasting which adopt OFDM having such characteristics as described above, the DVB-T2 standards, which are second generation European digital broadcasting standards, are available. The DVB-T2 standards are disclosed in “Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2),” DVB Document A122, June 2008 (hereinafter referred to as Non-Patent Document 1).
In the DVB-T2, data are transmitted in a unit of a transmission frame called T2 frame. Further, in the DVB-T2, a signal called FEF (Future Extension Frame) having a structure different from that of the T2 frame is multiplexed and transmitted with the T2 frame.
FIG. 1 illustrates a frame configuration of the DVB-T2.
Referring to FIG. 1, in the DVB-T2, a T2 frame and a FEF part are multiplexed for transmission. However, the FEF part is inserted only where this is required.
In what manner the FEF part is inserted is decided uniquely from the value of the FEF interval and the FEF length. The values of them are included in the L1 pre-signaling of a T2 frame of FIG. 2 hereinafter described. For example, where the value of the FEF interval is n and the value of the FEF length is m, one FEF part is inserted in n T2 frames, and the length of the FEF part is m samples. In other words, A=B=C=FEF interval (n).
FIG. 2 illustrates the format of a T2 frame.
Referring to FIG. 2, the T2 frame includes a P1 symbol, P2 symbols, and symbols called Normal and a symbol called FC (Frame Closing) (both of which are data symbols), disposed in this order.
It is to be noted that a portion denoted by GI in FIG. 2 represents the guard interval in an OFDM symbol, and the P1 symbol does not have the GI.
The P1 symbol is a symbol for transmitting the P1 signaling. The P1 symbol includes transmission parameters S1 and S2. The transmission parameters S1 and S2 represent in which one of methods of the SISO (Single Input Single Output (meaning one transmitting and one receiving antenna)) and the MISO (Multiple Input, Single Output (meaning multiple transmitting antennas but one receiving antenna) the P2 symbols are to be transmitted, the FFT size when FFT calculation of the P2 symbols is to be carried out, that is, the number of samples or symbols of an object of one cycle of FFT calculation, and so forth.
The P2 symbols are symbols for transmitting the L1 pre-signaling and the L1-post signaling. Further, since the P2 symbols include a greater number of pilots than ordinary symbols, utilization of the P2 symbols can raise the accuracy in various error detections which utilize pilots in comparison that of ordinary symbols.
The L1 pre-signaling includes information necessary to carry out decoding of the L1 post-signaling. The L1 post-signaling includes information necessary for accessing to a layer pipe of a physical layer.
Here, the L1 pre-signaling includes a pilot pattern (PP) representative of arrangement of a pilot signal regarding in which symbol or subcarrier a pilot signal which is a known signal is included, presence or absence (BWT_EXT) of extension of a transmission band for transmitting the OFDM signal, a number (NDSYM) of OFDM symbols included in one T2 frame, and so forth. The information included in the L1 pre-signaling is necessary for demodulation of the symbols of the data including the FC.
The L1 pre-signaling further includes information which represents such FEF sections as the FEF length and the FEF interval shown in FIG. 1 more accurately, and associated information representative of the type of the FEF such as the FEF_Type.
FIG. 3 illustrates a format of the FEF part. Referring to FIG. 3, the FEF part is undefined fully except that the maximum length thereof is 250 ms which is equal to that of the T2 frame and that the P1 symbol is placed at the top thereof. For example, also the average signal power may be different from that in the section of a T2 frame or no signal may be included. In other words, since it is not known whether or not the FEF part has a frame configuration, in the DVB-T2, the part is called FEF part. It is to be noted that, in the following description, the FEF part is sometimes referred to also simply as FEF.
Accordingly, although a reception apparatus at present need not acquire information included in the FEF except the P1 symbol, it has to detect that the FEF is inserted and operate such that reception of T2 frames is not influenced by the FEF.
In particular, the reception apparatus has to carry out P1 detection and estimate a section in which the FEF is inserted based on information included in the P1 and then operate such that a signal in the interval may not have an influence on ordinary reception of T2 frames for a period of time after reception is started until the L1 pre-signaling is acquired.
FIG. 4 shows a configuration of the P1 symbol.
Referring to FIG. 4, the P1 symbol intended in the DVB-T2 standards has the following ends:
a. The reception apparatus is enabled to decide early that the signal being received is a signal of the DVB-T2 standards;
b. The reception apparatus is enabled to decide that the preamble signal itself is the preamble signal of a frame of the DVB-T2 standards;
c. A transmission parameter necessary to start demodulation is transmitted; and
d. The reception apparatus can carry out position detection of a frame and correction of errors of the carrier.
As seen from FIG. 4, the P1 symbol has 1k (=1,024) symbols as effective symbols. The P1 symbol is structured such that a signal C obtained by frequency shifting part of the effective symbols A on the top side by a frequency fSH is copied on the front side of the effective symbols A and a signal B obtained by frequency shifting the remaining part of the effective symbols A by the frequency fSH is copied on the rear side of the effective symbol A. The frequency shift makes it less likely to detect an interfering signal as a P1 symbol in error on the standards.
The reception apparatus utilizes the fact that the P1 symbol includes a copy of part of the data thereof to determine a correlation value for each section to detect the P1 symbol. The detection of the P1 symbol is carried out, for example, upon initial scanning for checking which channel is used to transmit a signal of the DVB-T2 standards.
For the P1 symbol detected in this manner, fixed processes such as frequency correction, FFT calculation, CDS (Carrier Distribution Sequence) correlation calculation, scramble processing and DBPSK demodulation are carried out to decode the S1 and the S2 included in the P1 symbol.
FIGS. 5A and 5B illustrate transmission parameters of the S1 and the S2 included in the P1 symbol. It is to be noted that, in FIGS. 5A and 5B, X represents 0 or 1. The S1 is represented by a value of 3 bits as seen in FIG. 5A, and the S2 is represented by a value of 4 bits as seen in FIG. 5B.
When the S1 indicates 000, it represents that the received P1 symbol indicates a T2 frame of the SISO. When the S1 indicates 001, it represents that the received P1 symbol indicates a T2 frame of the MISO. When the S1 indicates 010, it represents that the received P1 symbol is not a preamble of a T2 frame. When the S1 indicates one of 011, 100, 101, 110 and 111, it represents the Reserved. In short, when the S1 indicates any other than 000 and 001, the received P1 symbol indicates a signal (FEF) with which the reception apparatus at present which only receives a T2 frame is not compatible.
When the LSB (Least Significant Bit) of the S2 is 0, it represents that the signal received is “Not Mixed,” but when the LSB of the S2 is 1, it represents that the signal received is “Mixed.” Here, the Not Mixed represents that the P1 of the signal being currently received is continuously same, and the Mixed represents that the P1 of the signal being currently received is different in each different frame and includes also the preamble of the T2 frame.
Accordingly, if the S1 and the S2 of a P1 symbol received at a certain point of time are checked, then the reception signal corresponds to one of the following patterns without fail:
A. A T2 frame is being received (S1: T2, S2: Not Mixed);
B. From within a multiplexed signal of T2 frames and FEFs, a T2 frame is being received (S1: T2, S2: Mixed);
C. Some other than a T2 frame is being received (S1: Not T2, S2: Not Mixed); and
D. From within a multiplexed signal of T2 frames and FEFs, a FEF is being received (S1: Not T2, S2: Mixed).
In short, by checking the S1 and the S2 of the P1 symbol, discrimination between the T2 frame and the FEF (T2/FEF) can be carried out.
Example of the Configuration of the Reception Apparatus
FIG. 6 is a block diagram showing an example of a configuration of a known reception apparatus.
Referring to FIG. 6, the reception apparatus 1 shown includes a resampler 11, a carrier frequency correction section 12, a P1 processing section 13, a GI correlation calculation section 14, a FFT calculation section 15, a fine error detection section 16, a coarse error detection section 17, a sampling error detection section 18, a correction control section 19, another correction control section 20, an equalization processing section 21, an error correction section 22 and a P2 processing section 23.
To an orthogonal demodulation section not shown of the reception apparatus 1, an IF (Intermediate Frequency) signal of an OFDM signal transmitted from a transmission apparatus is inputted. The orthogonal demodulation apparatus uses a carrier of a predetermined frequency, that is, of a carrier frequency, ideally a carrier same as that used in the transmission apparatus, and a signal orthogonal to the carrier to digitally orthogonally demodulate an OFDM signal inputted thereto. The orthogonal demodulation apparatus outputs an OFDM signal of a baseband obtained as a result of the digital orthogonal demodulation as a demodulation result.
The signal outputted as the demodulation result is a signal of a time domain before FFT calculation by the FFT calculation section 15 hereinafter described is carried out therefor, that is, a signal of a time domain immediately after a symbol, which is data transmitted by one subcarrier, on an IQ constellation is IFFT calculated on the transmission side.
The OFDM time domain signal outputted as the demodulation result is supplied to and converted into a digital signal by an A/D conversion section not shown and is then outputted to the resampler 11. The OFDM time domain signal is a complex signal represented by a complex number which includes a real axis component, that is, an I (In Phase) component, and an imaginary axis component, that is, a Q (Quadrature Phase) component. Therefore, circuit blocks to which a complex signal is inputted following the resampler 11 are indicated by two arrow marks. The resampler 11 finely adjusts the demodulation result in the form of a digital signal so that the sampling rate is synchronized with the clock of the transmission apparatus.
The carrier frequency correction section 12 carries out carrier frequency correction for the signal outputted from the resampler 11. A signal outputted from the carrier frequency correction section 12 is inputted to the P1 processing section 13, GI correlation calculation section 14 and FFT calculation section 15.
The P1 processing section 13 is a functional block which acquires the signal outputted from the carrier frequency correction section 12 and corresponding to an OFDM symbol of the P1 and carries out detection of a trigger position, a fine offset and a coarse offset and so forth. Further, the P1 processing section 13 can discriminate whether or not a signal being currently received is a T2 frame. A signal representative of the detected trigger signal is outputted to the FFT calculation section 15, and a detection value of the fine offset, also referred to as fine detection value, and a detection value of the coarse offset, also called coarse detection value, are outputted to the correction control section 19.
Here, the fine offset is an offset within an OFDM subcarrier interval which is fine while the coarse offset is an offset equal to an OFDM subcarrier interval which is coarse. In particular, correction with the fine offset is “finer” than that with the coarse offset, and correction with the coarse offset is “coarser” than that with the fine offset.
The GI correlation calculation section 14 acquires guard intervals from the signal outputted from the carrier frequency correction section 12 and uses the correlation of the guard intervals to detect the trigger position and the fine offset. A signal representative of the trigger position is outputted to the FFT calculation section 15, and the fine detection value is outputted to the correction control section 19.
The FFT calculation section 15 is a functional block which carries out FFT calculation for OFDM symbols based on signals supplied from the P1 processing section 13 and the GI correlation calculation section 14 and each representative of a trigger position. The FFT calculation section 15 extracts sample values of an OFDM time domain signal corresponding to the FFT size from the OFDM time domain signal in accordance with the trigger positions and carries out FFT calculation.
Consequently, from those symbols which configure one OFDM symbol included in the OFDM time domain signal, symbols of the effective symbol length with the symbols of the guard intervals removed are extracted as an OFDM time domain signal of the FFT interval and used for FFT calculation.
By the FFT calculation of the OFDM time domain signal by the FFT calculation section 15, the information transmitted from the subcarrier, that is, an OFDM signal representative of the symbols on the IQ constellation is obtained.
It is to be noted that the OFDM signal obtained by the FFT calculation of the OFDM time domain signal is a signal in a frequency domain and is hereinafter referred to also as OFDM frequency domain signal.
A result of the calculation of the FFT calculation section 15 is outputted to the equalization processing section 21, fine error detection section 16, coarse error detection section 17 and sampling error detection section 18.
The fine error detection section 16 uses an inter-symbol phase difference of OFDM pilots of the OFDM frequency domain signal obtained by the FFT calculation to newly detect a fine offset, and outputs a fine detection value to the correction control section 19.
The coarse error detection section 17 uses the fact that the modulation pattern of the OFDM pilots of the OFDM frequency domain signal obtained by the FFT calculation is known to newly detect a coarse offset, and outputs a coarse detection value to the correction control section 19.
It is to be noted that the fine detection value detected by the P1 processing section 13 is hereinafter referred to as P1-fine detection value, and the coarse detection value detected by the P1 processing section 13 is hereinafter referred to as P1-coarse detection value. The fine detection value detected by the GI correlation calculation section 14 is hereinafter referred to as GI-fine detection value. Further, the fine detection value detected by the fine error detection section 16 is hereinafter referred to as pilot-fine detection value, and the coarse detection value detected by the coarse error detection section 17 is hereinafter referred to as pilot-coarse detection value.
The sampling error detection section 18 detects a sampling error based on the OFDM frequency domain signal obtained by the FFT calculation and outputs an error detection value to the correction control section 20.
The correction control section 19 corrects an error of the P1-fine detection value from the P1 processing section 13 based on the GI-fine detection value from the GI correlation calculation section 14 and the pilot-fine detection value from the fine error detection section 16. Further, the correction control section 19 corrects an error of the P1-coarse detection value from the P1 processing section 13 based on the pilot-coarse detection value from the coarse error detection section 17. Then, the correction control section 19 generates a carrier frequency correction value by the correction of the detection values and outputs the carrier frequency correction value to the carrier frequency correction section 12.
The correction control section 20 controls operation of the resampler 11 based on the error detection value from the sampling error detection section 18.
The equalization processing section 21 carries out an equalization process in accordance with a characteristic of a transmission channel based on the pilot symbols included in the OFDM symbols of the OFDM frequency domain signal. For example, the equalization processing section 21 can carry out equalization of the signal transmitted thereto by carrying out complex division of the signal after the FFT calculation by an estimated transmission line characteristic. The signal equalized by the equalization processing section 21 is outputted to the error correction section 22.
The error correction section 22 carries out a deinterleave process for a signal interleaved by the transmission side and outputs a resulting signal to the P2 processing section 23 and a circuit on the succeeding stage.
The P2 processing section 23 acquires a signal corresponding to an OFDM symbol of the P2 and carries out decoding of the L1 pre-signaling and the L1 post-signaling. Information of the L1 pre-signaling and the L1 post-signaling obtained by the decoding is used for demodulation of symbols of data and so forth.